This application is related to co-pending application Ser. No. 808,939, filed June 22, 1977, in the names of Haven S. Kesling, Jr. and Lee R. Zehner, entitled PREPARATION OF UNSATURATED DIESTERS BY CATALYTIC OXIDATIVE CARBONYLATION OF DIOLEFINS, which application discloses certain aspects of the present invention.
In the above-co-pending application of Kesling and Zehner, which application is incorporated herein by reference, there is disclosed a process for the preparation of unsaturated diesters in general by oxidatively carbonylating with carbon monoxide and oxygen or an oxygen-containing gas in the presence of a platinum group metal catalyst compound, a copper or iron oxidant salt compound and a stoichiometric amount of a dehydrating agent at a pressure of between about 15 psig and 5000 psig and a temperature in the range of about 25.degree. C. to 200.degree. C., and alternatively contemplates the use of catalytic quantities of alcohols and ligands.
The present invention is directed to hydrogenation of the diester, dimethyl hex-3-endioate derived from the oxidative carbonylation of the diolefin, 1,3-butadiene, as disclosed in the above Kesling and Zehner application, and the conversion of the unsaturated diester to dimethyl adipate which dimethyl adipate may be hydrolyzed to adipic acid having a high degree of purity without resorting to the involved recrystallization procedures associated with prior art processes used to free adipic acid from various impurities especially if the adipic acid is to be employed to produce Nylon 66. The unsaturated diester, dimethyl hex-3-endioate, the adipic acid precursor, or the dimethyl adipate after hydrogenation of the diester may be distilled to remove impurities providing a more economical and efficient method for the preparation of high grade adipic acid by hydrolysis.
While oxidative carbonylation and hydrogenation as well as acid-catalyzed hydrolysis reactions are generally known, the prior art does not shown or describe a process for the oxidative carbonylation of 1,3-butadiene to prepare dimethyl hex-3-endioate and the catalytic hydrogenation thereof to dimethyl adipate, which may itself be employed as a plasticizer or lubricant.
A recent Japanese Kokai No. 75-130,714, Oct. 16, 1975, describes the preparation of carboxylic acid esters by reacting conjugated dienes, carbon monoxide and at least stoichiometric amounts and generally an excess of a monohydric alcohol in the presence of molecular oxygen and a Group 8 noble metal catalyst. Dehydrating agents may be used if necessary to maintain non-aqueous conditions. Very small amounts of the dimethyl hex-3-endioate are produced.